Late Mesozoic Depositional Features And Basin Evolution Of The Western Segment Of Bailongjiang Uplift Zone In The Songpan Area

Songpan area, adjacent to Longmenshan Orogenic Belt in southeast and'inverse-triangle'block gripped by Jinshajiang River and Garze-Litang fault belt, locates in the eastern margin of Tibet plateau, with the northern boundary of East Kunlun-West Qinling Orogenic Belt. The Songpan area is with the average altitude of 3500m and with the area of 120000 km2. Upper Triassic covers most of the region, except the eastern and southeastern area with Proterozoic, Paleozoic and Mesozoic cover. For the whole Tibet Plateau, this area is with good weather. And in the regional tectonics, Paleozoic is the passive continental margin dipping gently in to north with development of the basin-shelf facies black shales with more than 5000m. It developed into carbonate platform during late Paleozoic, covered by the large area of marine Triassic.The past survey on the oil resources of Songpan area always only focuses on the folding Triassic, ignoring the Jurassic-Cretaceous cap rocks of Bailongjiang uplift belt. Actually, through the systematic sedimentary study on the Jurassic-Cretaceous strata, the evolution and time of the regional tectonics movement could be reflected precisely and the deformation history and time of the Triassic could be controlled, providing certain scientific evidence for the study of oil reservoir conditions, oil resources prospecting and exploration work in Songpan area.The thesis study is based on the project of China Petroleum & Chemical Corporation, and chooses the late Mesozoic sedimentary features and basin evolution of the western segment of Bailongjiang Uplift Zone in the Songpan area as the main study content. It is on the basis of history study and the depositional study to conduct the systematic investigation of late Mesozoic sedimentary features and basin evolution.The analysis of architectural elements analysis of sedimentary environment shows that the depositional environment of ealy Jurassic in late Mesozoic basin (Gahai basin) in the study area is lacustrine environment, subdivided further into shorelake subfacies and shallow lake subfacies. The late Cretaceous is the alluvial fan-lacustrine environment, and the alluvial fan facies can be subdivided into root fan subfacies and mid-fan subfacies.Based on the analysis of depositional features and sedimentary environment of the Jurassic-Cretaceous strata in the study area, and the further association of new data of reflection seismic profile, structure, etc. from the source project, Gahai basin is strike-slip rifting basin with the main fault of Gahai-Jiacang-Langmusi sinistral strike-slip fault which began to act in Ladinian of Middle Triassic when the basin started to form. The depositional center locates in the mid-east part of the basin (Gongba-Langmusi area). The basin expanded during Jurassic to Cretaceous. Since Quaternary, along with the accretion of Tibet Plateau to north, the extent of Gahai basin decreased rapidly, and now the depositional center of Gahai basin is in the area of Gahai. The sedimentary basin was eroded at early stage, becoming the present frame of residual basin.